Long before you throw up, your body has already tried to protect you; your nose, tongue and eyes are the first guards that help you stay away from spoiled food. Throwing up is the back-up plan if something harmful gets swallowed, and food can carry many things your body wants to stay away from, such as germs, moulds and plant defence chemicals. Feeling sick and throwing up are ways the body responds when the gut meets one of them. Nausea and vomiting work as defences when harmful things get in, whether through the gut or through the blood. Throwing up, or vomiting, is the forceful removal of what is in the stomach and gut; nausea is thought to be a feeling, which is harder to study in animals. These are two different events: nausea is an abstract feeling of inner sickness, and it often, though not always, goes on to throwing up. Both are described as protective defence processes as of November 2026.

Throwing up can be started through four sensory routes: the digestive tract, the blood vessels, the cerebral cortex (the thinking and feeling part of the brain) and the balance system. Researchers think that some motion-sickness messages come in through the inner ear's balance system, while most other sick-feeling messages come from the gut and the blood. So the brainstem listens to several different senders at once. The brain's vestibular nuclei collect messages from the balance organs of the inner ear and the cerebellum, and these messages are part of what can set off motion sickness. Motion is sensed by the balance organs of the inner ear. These organs are wired to the brain's vomiting control, which is why some people feel sick on a boat, in a car or on a plane.

Special cells lining your gut make a chemical messenger called serotonin. When something harmful touches the gut, these cells can release it, and it can directly stimulate the ends of the vagus nerve to send a warning up to the brain. Most of the vagus nerve's sensory fibres carry a receptor for serotonin called 5-HT3. This is one reason serotonin from the gut is such an important part of the body's sick-to-the-stomach alarm, although scientists are still working out exactly where it acts. How the gut's lining cells decide that something is harmful enough to release serotonin is a key question. Some of these cells have sensors for sharp irritant chemicals, such as the TRPA1 channel, which can make them release it. Serotonin is not the only chemical messenger in the gut's warning system. Histamine, for example, works alongside serotonin in musk shrews, and scientists think no single messenger can explain every kind of sick-to-the-stomach signal. Spoiled food, called food poisoning, can make you throw up.

Part of what decides when you throw up is the brainstem. The vagus nerve passes gut messages to two places in the brainstem: the area postrema and a relay station called the nucleus of the solitary tract. That relay station also takes in news from the lungs and heart, so it handles many body messages, not only sickness. The area postrema sits in the brainstem where it can sense the blood directly. Because of this spot, it was historically called the chemoreceptor trigger zone, and it can pick up warning chemicals that have been swallowed and then travelled in the blood. An animal does not throw up when that zone alone is stimulated with electricity, which makes it only one piece of the circuit. Reaching from the brain, via the neck, out to the organs, the vagus is a long nerve.

Some brainstem relay cells connect to motor parts in the brainstem, including one that runs the vagus nerve. These networks likely hold key points for coordinating retching and vomiting, though scientists have not mapped them fully. Three separate digestive-tract parts make up emesis. These are the set-up in the stomach and throat, a backward giant contraction, and then the actions of retching and expulsion. A brainstem network runs each of them, and a central pattern generator coordinates the trio. The dorsal vagal complex, a group of brainstem structures, is in charge of the reflex that makes you throw up, and its main parts are the nucleus tractus solitarius and the area postrema. Input arrives at the complex from the vagus nerve that serves the gut, blood chemicals, the insular cortex and the balance nuclei that lie in the brainstem. Chemicals in the blood are sensed by the area postrema, an alarm for poisons that can begin the urge to vomit.

The vomiting center of the brainstem receives messages from four senders, namely the brain's thinking parts, the inner ear, the throat and the gut. As of January 2023, scientists have argued for decades about whether a single vomiting pattern generator exists, and some brain-stimulation and lesion studies said no, while others pointed to the ventrolateral reticular formation. In studies of the brainstem, stimulating a region called the Botzinger nucleus triggers retching and vomiting, which makes it a candidate for the vomiting central pattern generator. It is not settled whether the Botzinger nucleus controls all of throwing up or only the retch and vomit steps, because the studies did not record the gut responses. A 2023 review concluded that the gut's responses and their peripheral controls are well documented, but that much work remains on the central control mechanisms in the brain.

Few animals can vomit; non-emetic species include mice and rats, whereas dogs, cats, ferrets and shrews are among those that can, and in animals that can throw up, the reflex is a team effort. The belly muscles and diaphragm squeeze in rhythm, breathing and heart rate change, the gut wave runs backwards, the stomach empties more slowly and pressure inside the stomach rises. Mice may have a smaller, retch-like reflex. In some experiments, certain bacterial toxins and the switching on of particular brainstem cells made mice do a reduced version of retching, though how far this copies true throwing up is not fully clear. Scientists who study throwing up use animals that can do it, such as ferrets, shrews and dogs. But it is hard to use modern gene tools on these animals, so progress on how the reflex works inside the brain has been slower. Squeezing of the stomach by belly-wall muscles goes together with a forceful downward push from the diaphragm.

Throwing up is a very old process in the animal world. One science overview calls it a primitive process that has hardly changed as animals evolved, yet how the brain regulates it is still almost unknown. Scientists do not know whether animals that cannot throw up, such as rats, feel nausea the way people do. Nausea is a feeling, and there is no simple way to measure a feeling in a rat. Brain scans of people who feel sick show many parts of the brain at work, including parts for thinking and emotion. In one study, activity in two of these parts rose along with heart rate during the sick feeling. A separate study made volunteers motion sick with an optokinetic drum, which is a spinning striped drum, and recorded fMRI brain scans as they reported how sick they felt. The results suggest that many brain regions take part in the sick feeling, and big feelings, such as fear, disgust, worry or great excitement, can also make some people throw up.

Scientists do not yet know which vagus nerve cells are the ones that carry the sick feeling. The vagus has many types of sensory cell, some sensing stomach stretch and some sensing nutrients, but the ones for the sick feeling have not been identified. In animal experiments, cutting the vagus nerve to the gut abolished sick-feeling responses to several swallowed toxins, but not to motion. Researchers think the gut's vagus fibers help sense nausea messages from the gut. The brain's control of car sickness is not the same as its control of throwing up from something in the gut. Studies suggest the two use different brain pathways, even though they can end in the same vomit. When area postrema neurons of one type were removed, mice no longer learned to steer clear of cues tied to nausea-causing gut irritants such as lithium chloride.

Motion sickness is not only a human problem. It happens in both people and animals during travel by sea, car and plane, and even in space, and also in simulators, cinemas and video games. Cybersickness from video games likely comes from sensory conflict: your eyes see the world moving on the screen while your body sits still. As of November 2026, the leading scientific explanation of motion sickness is sensory conflict, where messages from the body, eyes and inner ears fail to match what the brain expects. The idea says the sickness comes from a clash between vestibular, visual and other input and what the brain expects from past experience, so it may happen when reports of movement from the eyes and inner ears disagree. Rides in cars and boats leave some people dizzy and queasy.

Sensory conflict alone cannot be the whole story; people adapt to motion even while conflicting inputs continue, so the idea has limits. Two main ideas about motion sickness exist as of November 2026, and the first, sensory conflict, holds that the senses disagree. The other, postural control, blames trouble adjusting posture, but little evidence supports it. Experts say we still do not fully understand the brain mechanisms behind motion sickness; people also differ a lot, and why some get it more than others is not clear. Some research says it peaks around nine or ten years old, and the longer a trip lasts and the bigger the movements, the worse motion sickness tends to be. A storm on a small boat for over eight hours would be much harder on the body than a smooth one-hour train ride.

Passengers often feel motion sick while drivers do not; this is probably because drivers anticipate the vehicle's motion better and move with it. Looking toward the road in front or toward the horizon can soothe car sickness, because the eyes then see the same movement the inner ears sense. Kids and adults throw up by the same basic steps, but kids trigger the reflex with less effort. A germ-caused stomach bug is the top reason kids throw up, and the bout usually ends within a day or two. Kids sometimes throw up after eating too much, when the stomach is stretched too full. Before throwing up you may go pale and your heart may beat faster, and your mouth can fill with extra spit, or saliva. If you feel sick, tell a grown-up, like your mom or dad; a grown-up can check on you and get a doctor if you need one.

Words to know

• emesis: vomiting
• vestibular: inner-ear balance
• nucleus: brain cell cluster
• sensory: about the senses
• brainstem: base of brain